Download Electrical ground probing

Earthlearningidea - http://www.earthlearningidea.com
Electrical ground probing
Measuring the electrical resistance of the ground to find buried objects
Pupils may be familiar with the principle of using
geophysical methods to locate objects and
structures buried below ground. Some TV
archaeology programmes refer to this as
“Geophys”. The electrical resistance of the ground
is one property which can be used in this way.
Set up the apparatus as shown in the diagram
and photograph. A current is passed from a
voltage generator into a beaker of sand, through
two electrodes (steel pins), mounted in a piece of
card, (so that they are held about 3cm apart).
Measure the voltage and current across the
electrodes. For the first run, use sand that has
been soaked in saturated brine. Adjust the voltage
generator so that a voltage of between 2 and 4
volts, and a current of about 100 mA are shown
between the electrodes. Record the two values.
Then change the beaker for one which contains
sand that has been soaked in deionised water,
and without changing the setting on the generator,
measure the voltage and current between the
electrodes and record the values. Finally, change
the beaker for one containing dry sand and repeat
the measurements. Ask the pupils to calculate the
resistance of the contents of each beaker, using
Ohm’s Law (V = IR, so R = V/I).
Circuit diagram of the apparatus
The table below shows figures obtained during a
pilot run.
Sample
Dry sand
Sand
soaked in
deionised
water
Sand
soaked in
salt water
Voltage
(V volts)
4.76
3.16
Current
(I amps)
0
0.01
Resistance
(R ohms)
Infinity
316
2.86
0.45
6.3
The apparatus set up to measure the resistance of the sand
Ask students to predict what would happen to the
resistance if an iron bar were to be buried
horizontally in the sand, parallel and close to the
bases of the electrodes (as shown below). Set up
such a model, using the brine-saturated sand and
repeat the activity. (Caution – do not let this run
for too long, since chlorine is eventually produced
at one of the electrodes).
Electrodes
The way electrical methods are used in the field – in this case,
to locate a buried steel drum
(Photographs by Peter Kennett)
Buried bar
Plan view
1
Earthlearningidea - http://www.earthlearningidea.com
The back up
•
Subtitle: Measuring the electrical resistance of
the ground to find buried objects
•
Title: Electrical ground probing
•
Topic: A laboratory demonstration of the principle
of ‘remote sensing’, using the electrical properties
of Earth materials.
Age range of pupils: 14 – 18 years
•
Time needed to complete activity: 20 minutes
plus setting up time
•
Pupil learning outcomes: Pupils can:
• measure a voltage and a current passing
through a substance;
• calculate the resistance using Ohm’s Law;
• explain why the resistance is less for sand
saturated in brine than for sand saturated in
distilled water or dry sand;
• predict the change in resistance when an iron
object is buried within the sand;
• discuss how such methods could be used to
locate buried objects, such as a mineral vein or
a hidden cache of illegal weapons.
The brine-soaked sand has the lowest
resistance because the water is rich in ions
from the sodium chloride.
An iron bar has the least resistance of any of
the materials used in the activity.
In the field, mineral veins are usually of low
resistance, not so much because of the metal
ores present, but from the ion-rich water
trapped between the particles of the
weathered material above the vein.
Buried steel objects, such as weapons, can
be expected to reduce the resistance of the
soil above them.
For simplicity and in order to use familiar
concepts, this activity measures the
resistance of the materials and keeps the
same spacing and depth for the electrodes. In
the field, the spacing of the electrodes is
deliberately varied to obtain more information.
In this case, it is the resistivity of the medium
which is calculated (in ohm metres).
Resistivity may be regarded as the opposite
of conductivity.
Thinking skill development:
Pupils establish a pattern from the successive
measurements. These do not always work out as
expected, involving cognitive conflict. Applying the
model to the real world of remote sensing is a
bridging skill.
Context: The activity models the principles
involved in surveying by electrical methods. Such
techniques are frequently used in mineral
exploration or in archaeological surveying.
Forensic scientists also use the method to
investigate disturbed ground in the search for
objects buried by criminals.
Resource list:
3
• 3 sand samples (400 cm dry) in three 500 ml
beakers
• 150ml deionised water
• 150ml saturated brine (salt water)
• power pack
• ammeter
• 2 steel electrodes (mounted 3cm apart)
• volt and amp meters
• 5 connecting leads and crocodile clips
• iron bar about 2.5cm long
In the final set up, the buried iron bar significantly
reduces the resistance because it provides a low
resistance path in parallel to the direct path
between the electrodes.
Following up the activity:
Pupils could be invited to watch TV! Several
archaeology series (in U.K.) demonstrate the use
of geophysical methods in locating buried objects.
Source: Based on an activity developed for the
workshop “Sensing the Earth: teaching KS4
Physics”, Earth Science Education Unit,
http://www.earthscienceeducation.com
Underlying principles:
•
In order to conduct electricity, free electrons
or ions must be present to carry the current.
•
The dry sand has the highest resistance
because such ions are not present.
 Earthlearningidea team. The Earthlearningidea team seeks to produce a teaching idea regularly, at minimal cost, with minimal
resources, for teacher educators and teachers of Earth science through school-level geography or science, with an online discussion
around every idea in order to develop a global support network. ‘Earthlearningidea’ has little funding and is produced largely by
voluntary effort.
Copyright is waived for original material contained in this activity if it is required for use within the laboratory or classroom. Copyright
material contained herein from other publishers rests with them. Any organisation wishing to use this material should contact the
Earthlearningidea team.
Every effort has been made to locate and contact copyright holders of materials included in this activity in order to obtain their
permission. Please contact us if, however, you believe your copyright is being infringed: we welcome any information that will help us to
update our records.
If you have any difficulty with the readability of these documents, please contact the Earthlearningidea team for further help.
Contact the Earthlearningidea team at: [email protected]
2